A Swarm of Probes to the Stars

byPaul GilsteronAugust 1, 2011

Just how big does a spacecraft have to be to do productive work? It’s a provocative question in this era of CubeSats and downsized budgets, but when you start thinking interstellar, there are even more reasons to wonder how small you can make your vehicle. After all, the propulsion challenges facing interstellar missions are profound, and the more mass (including, of course, the fuel you’re carrying), the greater the problem. The rocket equation is telling, and one of the many things it tells us is that trying to reach a nearby star with a conventional rocket is out of the question, at least if you’re hoping to get your mission there in a reasonable century or so.

Mason Peck has been pondering issues like this for a long time. Those of you who are unfamiliar with his work at Cornell can catch up by reading earlier Centauri Dreamsposts, but a more current solution is the article Exploring Space with Chip-sized Satellites, which runs in the current issue of IEEE Spectrum. In it, Peck looks at the way our probes have grown over the years, from the Herschel Space Observatory’s 3.5-meter telescope to Cassini’s 11-meter magnetometer boom, and goes on to discuss a far different idea: The creation of spacecraft the size of dust motes, probes that operate in swarms, with not just interplanetary but interstellar reach.

Spacecraft on a Chip

So instead of the Enterprise, let’s imagine a microchip 20 micrometers thick and weighing 7.5 milligrams. We’re talking about tiny solar sails — Peck calls these ‘Sprites’ — that would have the right ratio of surface area to volume to accelerate at about 0.06 mm/s2, making them about ten times faster than the Japanese IKAROS sail. Now we’re in range of interplanetary missions for tiny spacecraft that can carry a single sensor each. Think of each ‘chip probe,’ though, as just one node that operates along with a multitude of such chips to form a distributed sensor network in space.

Here’s one local use, as Peck explains in his IEEE Spectrum article:

We could, for instance, send tens of thousands of Sprites into orbits between Earth and the sun. These simple chips would have one task: to send a signal to Earth when the local magnetic field or the number of charged particles that hit the spacecraft exceeded some threshold. Taken alone, each chip would provide just one data point. But a network of these scattered chips could produce 3-D snapshots of space weather, something no traditional spacecraft, no matter how sophisticated, could ever do on its own. A payload of a million of the relatively heavy 50-mg version of the Sprite would amount to just 50 kilograms, about the mass of a single science instrument on one of NASA’s larger interplanetary spacecraft. So the launch costs of a Sprite network would be significantly lower than that of a traditional satellite.

Peck is working on prototypes for such chips, about which more in a moment, but Centauri Dreams is drawn to the interstellar consequences of his work in the long-term. And on that score, today’s small chip prototypes point to an interesting propulsion option. For the Sprites are of such low mass that they should be able to take advantage of the magnetic fields around planets. The Lorentz force affects charged particles that move through a magnetic field, an effect that shows up in the famous ‘spokes’ that cut across Saturn’s rings, the result of the ionization of dust particles by virtue of sunlight and plasma effects and the consequent forces of the planet’s own magnetic fields on the ionized dust.

Now think about harnessing the Lorentz force around a still larger Solar System planet. Jupiter’s magnetic field is 20,000 times stronger than the Earth’s, and could be used as a natural particle accelerator. To do this, each Sprite would need an electric charge, easy enough to create with the help of plasma in a planetary environment (Peck has been testing the process at Cornell by exposing Sprite-sized objects to a stream of xenon plasma). A charged Sprite would use the Jovian magnetic field to accelerate itself in orbit, reaching thousands of kilometers per second, before turning off its onboard power supply, at which point the Lorentz force disappears and the spacecraft is hurled at high velocity out of the Solar System.

Time to Alpha Centauri? Perhaps a few hundred years, but that’s much faster than the best solar sail ideas, which have trouble reaching below a thousand years for the journey. And as with solar sails, we can also consider the possibility of using a laser assist to power the tiny sails to much higher velocities. Ten percent of the speed of light becomes possible in this scenario, and that gets you to Alpha Centauri space in a more than manageable forty-plus years.

All kinds of questions crop up in this work, not the least of which is the communications capability of the Sprite swarm, and issues of the environment the Sprites move through:

Sprites will also be far more vulnerable to damage than present-day spacecraft. Because they are lightweight and have solar cells built directly on the chip, Sprites can’t be equipped with radiation shielding to protect their electronics. This lack of shielding also makes the chip more vulnerable to impacts with micrometeorites, which zip at high speed throughout the solar system. Sprites could compensate for these hazards through sheer numbers; missions could be designed so that a significant fraction of the chips could be lost without dooming the operation.

But issues like these are also what Peck is working on both at Cornell and through experimentation at the International Space Station. There, Sprite prototypes have been mounted on the exterior of the ISS, each containing solar cells, a microprocessor with radio, an antenna and amplifier and the necessary switching circuitry. With little power to work with, the transmitters will produce only the weakest of signals, so the current work focuses on code-division multiple access (CDMA) techniques to help pull signal out of the noise.

Image: There are currently two Sprite prototypes being tested: a Printed Circuit Board version and a Multi-Chip Module developed through a collaboration with Sandia National Laboratories. These are both traceable to an objective ‘System on Chip’ prototype. Credit: Mason Peck/Cornell University.

A Future in the Swarm

The overall project looks at the physics of tiny objects and the uses of mass production, and the concept echoes similar notions in play in the computing community:

The idea goes back at least 15 years, and it has its origins in “smart dust”—tiny microelectromechanical sensor systems that can be used to measure light and temperature, register movement and location, and detect chemical and biological substances. The notion of sending such systems into space was slow to gain momentum. But it began to take off once space researchers realized that integrated circuits had become quite inexpensive, dense, and easy to fabricate, and that almost everything a spacecraft needs can now be made with semiconductors alone: solar cells for power, capacitors for energy storage, and all the memory and processing capability you could want. As with smart dust, a diversity of payloads can be fabricated to ride on a chip, including basic spectrometers, load sensors to measure particle impacts, chemical sensors, and simple CMOS cameras. Researchers around the world, including groups at Surrey Space Centre, the University of Strathclyde, the Aerospace Corp., and the Jet Propulsion Laboratory, have explored the possibility of making chip-based spacecraft and are investigating their capabilities.

What a notion. We can couple these ideas with hoped-for advances in nanotechnology that may take Sprite concepts down to far smaller scales. I remember talking to Robert Freitas when writing Centauri Dreams (the book) about his idea of intelligent ‘needle’ probes, tiny interstellar craft spewed into the deep by the millions, each containing the assembler technology that would allow it to harvest local materials in a planetary system to build the necessary scientific station upon arrival.

Mason Peck’s Sprites would seem to have real possibilities here in the Solar System (imagine a swarm floating in the clouds of Titan to provide ongoing analysis of that distant world), and we can only speculate about their possibilities for interstellar missions. But moving out of the era of the huge space vehicle with gigantic fuel tanks into the realm of the miniature takes us in a direction that offers real potential for the kind of propulsion that could reach a nearby star. And as our work at nanotechnological scales advances in coming decades, we may learn how to solve some of the seemingly intractable communications problems such probes would face.

If we use these within the solar system, especially in orbit around any body, we first need a way to kill the swarm on mass by getting them to de-orbit. Otherwise the clouds of “shrapnel” will impede future exploration of that body.

Also (and I have not read the paper) the probe/chip needs protection of the micro-circuitry from not only cosmic rays but even from the solar wind. This is already a problem even when embedded inside a large, bulky metal satellite. That could add to the weight unless a dielectric shell (to avoid impeding communication and data collection) can be made light and robust enough to give the needed protection for the life of the mission, including long enough to receive and act on the previously-mentioned kill signal.

A disappointing concept for those of us who want to see manned interplanetary and interstellar missions, but extremely practical and exciting given the direction of our technology for the last thirty years. This summary of Peck’s ideas is appreciated.

Let’s hope to see some experimental payloads lofted to cislunar or interplanetary orbits within the decade!

Miniaturization of space probes does make sense, considering how the underlying semiconductor and MEMS technologies continue to miniaturize. It would reduce cost as well. Large numbers of these probes can be made in a MEMS foundry and launched by a single, small launcher. Scientific exploration of the solar system on the cheap. I can think of a commercial application this would be really good for, prospecting the asteroids for PGM’s (Platinum Group Metals).

Admittedly, short of something like Clarke’s Seedships (sure, we’ll upload your genome to a Sprite for the right price), these ChipSats are not the short-term solution to extending human presence beyond Earth. However, maybe they’re on the long-term path. In fact, I’d argue that fingernail-size spacecraft democratize space exploration in a way that even CubeSats don’t. For about $100K, you can build and launch a 1 kg CubeSat, but even that relatively modest cost is out of reach for nearly everyone. Instead, maybe Sprites are the technological wedge that pries open interplanetary–and interstellar–space.
As little as they would cost, Sprites have the potential to develop a user community–the ‘Altair users’ of space exploration. Taking this well-worn personal-computer analogy a little further, let me remind you that you don’t start your planet’s computer revolution with the internet, Google, eBay, and YouTube in place. And you don’t get there with Univac mainframes. You start with the Altair, move on to the Apple ][, work your way through PCs and Macs, and end up 30 years later with IPads and global connectivity.
We can do more with personal computers than with mainframes, and what we can do is qualitatively different. Similarly, Sprites offer new flight mechanics, ways to exploit space physics that may make extrasolar missions more feasible. These mission architectures will be new. They won’t resemble Voyager or Hubble. They will not feature exquisite, multimillion-dollar science instruments. They won’t send gigabytes of data to an astronomer at a university to pore over for decades. Instead, they will be about getting there soon, and doing so with a quantum of spacecraft technology. We might launch them out of railguns or particle colliders. Maybe we will explode a warhead’s worth of energy from within a dense block of Sprites, sending explorers-as-shrapnel in all directions, at varying speeds (some very fast), for a kind of stochastic survey of the galaxy. And we won’t care if most of them fail because of high-energy solar particles, cosmic rays, or even poor fabrication. The number of Sprites at the beginning of a mission will be based on the failure statistics, exploiting a sweet spot where the cost benefits of fabricating many chips meet adequately reliable performance.
I count on my colleagues in the nerd community to increment what a ‘spacecraft quantum’ consists of: build on our open-source Sprite idea. Find the killer apps and make tiny explorers commercially viable. At some point in the future, we may see the crescendo of technology that takes people into space. NASA has had its successes with its mainframe-like flagship missions. From the look of things, maybe that’s as far as they’ll get. Now let’s see what the rest of us can do.

Very interesting and promising stuff. Here is a dumb question: as I recall from freshman physics, magnetic fields do no work (F = q v x B). So how would the Lorentz force of a planet’s magnetic field accelerate these probes?

Just to be clear, that is the marginal cost, not the actual cost. It assumes hitching a ride on a launch vehicle, paid for by someone else, that just happens to be headed on the desired trajectory. Doing the launch on your own of even one grain of sand is a wee bit more expensive.

I’ve been wondering for a few months now whether such “sprite” probes could be accelerated with a large apparatus in orbit; something like a cross between a rail gun and a CERN-type particle accelerator. Granted, accelerating a milligram mass is a bit trickier than a proton, but maybe at a Lagrange point, a much bigger ring than Fermilab or CERN could be feasible. Once operating, the ring could accelerate a series of sprites toward an interesting target, allowing observation over a lengthy time, even if each sprite’s passage is over in a matter of minutes.

For communication, I imagine two possibilities:

1) Bucket Brigade: If the series of sprites is heading in the same direction, data from earlier once could be passed via laser or radio back to the more recently-launched ones, in a peer-to-peer fashion, greatly reducing the range any one of them would need to transmit.

2) Quantum entanglement: a large set of pairs of particles is prepared with entangled quantum states. One of each pair is loaded onto the sprite prior to launch, and one of each pair kept back at “base”. Each time a bit is needed to be communicated, one of the entangled particles on the sprite is “spent” to affect the corresponding one back at base. Of course, error detection and correction bits would be needed (like on a CD recording) and it might be prudent to have the sprite’s pool of communication bits included particles entangled with multiple base stations for redundancy sake. This would remove distance as an obstacle, but separating, maintaining, then using very large numbers of entangled particles would have a _lot_ of practical issues. Also, once your entangled bits are all spent, no more communications!

Perhaps these sprites could be bacterial sized ( 1 micron) . While the bacteria may not be alive in the reproductive sense, bacteria-like particles could have ideal properties, self repair is possible, they may utilize an internal biochemical power source, they can use biomolecules for sensors, they might alter their florescence to communicate bits and bytes . Conceivably they would only have to accommodate a low water environment (many bacteria are dessication resistant) . They would be very cheap to “grow” utilize a three dimensional design ( instead of a flat ship) Perhaps life on earth is the result of some interstellar micro probes escaping built-in restraints and going ” native”. Think of it! Paul ,Kurt9,LKB and the rest of us might all be “a son of a probe…”
(Or daughter.. not sexist here for the ladies in the audience)

The Cosmist: “…So how would the Lorentz force of a planet’s magnetic field accelerate these probes?”

Great point. The article may not be clear in this regard. The Lorentz force only serves as a planetary-scale trap for the spacecraft, keeping it in the solar system while some external power source pumps up its kinetic energy. In that respect, this setup resembles a cyclotron, where charged particles accelerate thanks to the voltage across a gap through which the particles pass.

This concept addresses one of the general principles that makes interplanetary travel hard: most of the resources you need are available only at the start of the journey, i.e. in the solar system. At least, that’s true for the technologies we know about. Beyond a certain distance, you’ve got to coast until you arrive. Holding the spacecraft at Jupiter gets around that problem because we can fire lasers at the chips to accelerate them or let them accelerate like solar sails thanks to solar pressure, for example.

“What concerns me about this is the possibility of creating more space junk that will have to be avoided.”

Another great point. Fortunately, physics comes to the rescue here. Sprites in low Earth orbit reenter very quickly. The reason is that their small scale leads to very low ballistic coefficients (low ratio of inertial forces to drag forces). For instance, a Sprite released near ISS reenters in a few hours. The same principle actually may help these little explorers flutter down through a planetary atmosphere, doing science without burning up. So, I would argue that they represent a vanishingly low risk in LEO.

1. There are obvious potential pros and cons for this technique. There are limits to the type of experiments that can be done. For example, I can’t see how a mass spec could be run using these chips. So the approach must be complementary to macro probes. Having said that, teh approach clearly plays to the strengths of increasing miniaturization, and “cheap, good enough” technologies.

2. So how would we know if earth (or the solar system) was a target of this type of interstellar probe? If probes were dust grain or microbe in size, wouldn’t they be all but undetectable? Would hey need some large reciver for their data parked nearby?

We can create magnetic fields much stronger than Jupiter’s’ magnetic field, why not simply put a one of these 45 Tesla magnets in orbit, powered by the sun?! The thing I don’t understand is how these probes would maneuver? This sounds more like a simple shoot in the direction of a star and hope it hits something of interest. Which would make it highly inefficient, you would most likely need billions if not trillions of these per solar system.
Now these would be useful in Sol system for certain.

Very, very well said, Mason Peck. You make an excellent point about the future of space exploration, which parallels in many ways with METI and SETI.

The days of big ships/mainframes probably are coming to an end, or at least a big reduction, as technology becomes smaller and more efficient, along with cheaper and more accessible to all in the process. Certainly we need to start changing our past views and paradigms of how we explore deep space and send/receive messages between the stars based on current and projected future technologies.

Imagine if and when many people and groups are able to send literally swarms of probes throughout the Milky Way galaxy, rather than hope that one day some group with tons of resources and money can get one big probe together.

This may be yet another answer to the Fermi Paradox: ETI is here, in droves, they are just very small. An excellent way to study a less advanced species as unobtrusively as possible. Makes one wonder even more about those reports of massive spacecraft with crews of big-headed, almond-eyes aliens who want to interbreed with us.

On a related note, Joe Davis of MIT has written about microbes like bacteria being spread across the galaxy via stellar winds either individually or attached to dust carrying messages in their DNA. Maybe we need to start capturing more interstellar dust and examining it very closely. Again, we should take the retirement of the Space Shuttle as a symbol and motive for changing how we and others may do things out there.

ljk: “This may be yet another answer to the Fermi Paradox: ETI is here, in droves, they are just very small. ”

Douglas Adams: “…For thousands more years the mighty ships tore across the empty wastes of space and finally dived screaming onto the first planet they came across – which happened to be the Earth – where due to a terrible miscalculation of scale the entire battle fleet was accidentally swallowed by a small dog.”

Nice to see you here at CD, Mason. Just how fast do you think the chip-sats can get while in Jupiter’s influence and what kind of energy input does it require? I’m interested in applying chip-sats to the task of shooting fuel and/or momentum to a larger vehicle, which Gerald Nordley has suggested elsewhere.

@danvk:
“And as our work at nanotechnological scales advances in coming decades, we may learn how to solve some of the seemingly intractable communications problems such probes would face.” — clearly, nobody has any idea how to achieve this.

Your sprite has landed on an Earth-like exoplanet. How does it take a photo of the landscape, and how does it transmit that photo back to the Solar System? How does it test for life, microbial or metazoan?

We could have an interesting side discussion about these microscopic spacecraft at the upcoming worldships symposium later this month — whose focus will be on the largest mainframe vehicles the mind can conceive, with masses of billions of tonnes apiece!

1) What is the mean time before failure of an unprotected 20 micron chip in the interplanetary environment?

2) How much communication power can a tiny chip like that produce and how far would it be able to send discernable signals?

As long as these are not addressed in a quantitative manner, this idea is just a flight of fancy. Perhaps they have been, but I am not aware of this and only passing mention is given to these problems in the above.

These could be difficult hurdles even in interplanetary space and at orbital velocities. They would be almost infinitely more challenging for interstellar purposes.

Redundancy is not the answer to 1), because the exponential decay kills you just as mercilessly as the rocket equation: If after one month half are still left, after 2 years it will be 1 out of 16 million. Relay might work for 2), but then we are talking a continuous stream, not a compact swarm. This is the right way to go, but a continuous stream of microchips is not an inexpensive solution, and a single break can doom the entire stream.

I like the Lorentz force idea. It goes with charge per mass, i.e. voltage, so it should be applicable to large probes as well as small ones. Or not? In space it should be possible to accumulate charge in the mega-, perhaps even gigavolt regime using an accelerator emitting a charged relativistic beam (Not on a chip, though…). Electrons would be best, as they minimize mass loss and are automatically recycled. That and a light sail (or perhaps the charged beam itself?) should get you to pretty high speeds around Jupiter by gradual acceleration in a Lorentz-forced orbit.

It may also be worth considering solar orbit. The field is weaker, but the radius much larger, which should compensate. Plus, you could go closer to the sun where there is more power.

@Eniac, in regards to point #1, most chips that are used in planetary probes are hardened against radiation, using a sapphire or silicon oxide substrate. I would imagine they would use the same techniques. If they have redundancies built in (ie. multi-core), this would also harden the chip against faults.

It is certainly an idea worth trying. But I don’t think it is even remotely possible that those sprites can survive an interstellar journey. One thing. Radiation. And a lot of it. And no shielding. And microscopic circuits. No good. And you can’t even shorten the exposure by flying faster because faster means more energetic collisions with the interstellar medium itself, which causes more radiation damage.

I would bet, without knowing, that those chips are not only hardened, but also shielded. The latter could not be done for sprites, and the question that needs to be answered is how long the MTBF can be with only the former.

Shielding in probes are small, due to weight constraints and size limitations. Any type of shielding has to be thick and heavy to absorb not only primary radiation but secondary radiation as well. Companies work more on hardening the chips against radiation. Again even hardening will have to increase logarithmically to protect a sprite in interstellar space, it would take some advancements, possibly a form of doped diamond and/or photonic circuits.

I am not an expert, but I imagine that even a mm layer of aluminum or ceramics can help protect a chip from a lot of radiation that would otherwise damage it. Alpha radiation, for example, and X-rays. For this reason, I do not think chips are left bare in space probes. I bet they are well packaged, housed, and positioned towards the interior for protection. What I am saying is that we should not get too excited until we have at least a rough quantitative estimate of MTBF for naked, ultrathin chips in space.

It appears that the effect is much too small to be of significant value for true propulsion. From the conclusions:

Material limits constrain the maximum charge per mass that can be achieved. For the best material available, and with a sphere size of 3m radius, q/m=0.03 C/kg seems to be the maximum. For this limit, earth escape and drag compensation are out of the question. However, less-demanding scenarios, such as formations and Jupiter insertion, are well within material capabilities. In that case, less efficient but less exotic materials, like VDA Kapton, may even be used for the Faraday cage. If a more efficient charge-storage system than a conductive sphere can be developed, this constraint might be relaxed, and remarkable feats such as earth escape without propellant may be realized.

In particular, there does not seem to be room for the forced Lorentz orbit with super-high velocities that was mentioned in the post:

A charged Sprite would use the Jovian magnetic field to accelerate itself in orbit, reaching thousands of kilometers per second, before turning off its onboard power supply, at which point the Lorentz force disappears and the spacecraft is hurled at high velocity out of the Solar System.

Of course, this apparent discrepancy is likely Peck vs. Peck, so I would be interested in any link to an analysis that bridges the gap.

Perhaps much smaller spacecraft can do better, but then there are other steep challenges, such as miniaturizing high voltage components for the electron accelerator needed for charging. Also, a spherical shape being optimal, it seems chips with corners and edges would be at a decisive disadvantage.

Mason Peck, associate professor of mechanical and aerospace engineering, has been named NASA’s chief technologist, effective January 2012. Peck will serve as the agency’s principal adviser and advocate on matters of technology policy and programs.

Peck leads several Cornell spacecraft research programs including CUSat, an in-orbit inspection system consisting of a pair of twin satellites designed and built at Cornell. CUSat is scheduled to launch in 2013 on a Falcon 9 rocket through the U.S. Air Force Research Laboratory’s University Nanosatellite Program.

Peck also is principal investigator of the Violet satellite experiment, also a Cornell-built system that will provide an orbiting test bed for investigating better commercial Earth-imaging satellites. Violet carries an ultraviolet spectrometer that will be used as a precursor to understanding exoplanet atmospheres.

In his NASA role, Peck will help communicate how NASA technologies benefit space missions and the day-to-day lives of Americans. The office coordinates, tracks and integrates technology investments across the agency and works to infuse innovative discoveries into future missions.

In addition, Peck will lead NASA technology transfer and technology commercialization efforts, facilitate internal creativity and innovation, and work directly with other government agencies, the commercial aerospace community and academia.

Peck will serve in the position through an intergovernmental personnel agreement with Cornell, where he will continue as a faculty member.

At Cornell, Peck’s work focuses on spacecraft dynamics, control and mission architectures. His research includes microscale flight dynamics, gyroscopic robotics and magnetically controlled spacecraft, most of which have been demonstrated on NASA microgravity flights.

He has worked with NASA as an engineer on a variety of technology programs, including the Tracking and Data Relay Satellite System and Geostationary Operational Environmental Satellites. The NASA Institute for Advanced Concepts sponsored his academic research in modular spacecraft architectures and propellant-less propulsion, and the International Space Station currently hosts his research group’s flight experiment in microchip-sized spacecraft.

As an engineer and consultant in the aerospace industry, he has worked with organizations including Boeing, Honeywell, Northrop Grumman, Goodrich and Lockheed Martin. He has authored 82 academic articles and holds 17 patents in the U.S. and European Union.

Peck earned a doctorate in aerospace engineering from the University of California- Los Angeles as a Howard Hughes fellow and a master’s degree in English literature from the University of Chicago.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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